Possible Clues:

• Threatening phrase cryptically found in Elsinore
• ". . . if you want to avoid trouble"
• Choice words?
• Words often accompanied by a 45-Across
• Them's fighting words
• Warning head will be sacked from central London school
• Unspoken part of the Godfather's "offers"?
• Common threat
• Bully's last words
• Part of a stern warning